Although a nuclear spin-diffusion process through magnetic dipole-dipole interactions could cause a single exponential recovery curve, the large contribution from the spin diffusion is excluded, because its isotopic ratio of 1.28 [1]} is larger than the ratio of 1.168 in the pure quadrupole relaxation and the experimental ratios in Fig. REF  (lower panel).
One may infer the mechanism in \(^{63}W_M\)  due to dilute electron and hole scatterings in semiconductors [2]}.
The exchange split energy between the orthoexciton state and the paraexciton state is 12 meV [3]}.
The magnetic energy gap \(\it \Delta \)  may be associated with the low exchange energy of the ortho-to-paraexcitons.
The other energy scales like an exciton binding energy and the band gaps are far larger than \(\it \Delta \)  [4]}, [5]}.
